In wireless systems, if the coverage of a first basic service set (BSS-1) overlaps a part of the coverage of a second BSS-2, the two BSSs are called overlapping BSS (OBSS). When any of the access points (APs) or stations (STAs) tries to access the channels for wireless transmission, it performs a channel sensing and a random back-off contention procedure. Any station or access point that wins the contention can transmit into the wireless medium to gain transmit opportunities (TXOP), and other stations or access points that sense the channel is busy will defer their transmission until the TXOP is over. Such carrier sense multiple access (CSMA) wireless protocol minimizes the likelihood of collision in which more than one stations or access points transmit into the wireless medium causing the reception at the intended recipient to fail. In general, the CSMA protocol is intended for devices (station or access point) with omni-direction antennas such that all the devices sharing the wireless medium can hear one another. However, if one device cannot hear another device in the same wireless medium, the device is called a hidden node.
With a hidden node in the wireless network, potential collision can occur since the hidden device is unable to sense the transmission of other devices. Note that since the two wireless networks (e.g. BSS-1 and BSS-2 in this example) are sharing the same wireless medium for transmission, the achievable throughput of each individual network is reduced whereas the aggregate network throughput of the two wireless networks remains approximately the same. For long-range outdoor networks, the increased coverage range results in many overlapping wireless networks within the same area, thus leading to significant reduction in the throughput of each individual network. Therefore, there is a need to manage the communication in wireless networks with overlapping coverage.
Sectorization is a powerful technique commonly used in outdoor wireless networks. The partition of the coverage area of a BSS into sectors, each containing a subset of stations, is called sectorization. In a mobile-to-mobile communication context, the partition of a field of view of interest or a coverage area of a station into antenna sectors is called sectorization. This partition is generally achieved by the AP transmitting or receiving through a set of antennas or a set of synthesized antenna beams to cover different sectors of the BSS. The goal of sectorization is to reduce medium contention or interference by the reduced number of stations within a sector. During sectorized beam transmission, SO (spatially orthogonal) OBSS STAs and APs can receive the omni transmission but not the sectorized beam transmission from the OBSS APs and STAs. By dividing the coverage area into multiple sectors using sectorized beams, the hidden node issue can be mitigated and the overall network capacity increases due to more efficient spatial reuse.
Multi-user Multiple-input Multiple-output (MU-MIMO) is becoming a system technique to enable high system capacity in both IEEE 802 and 3GPP LTE standards. For spatially separated STAs, downlink (DL) MU-MIMO transmission transmits to multiple stations simultaneously. Unlike SU-MIMO, the MU-MIMO can be used in a LOS propagation while SU-MIMO is restricted to multipath propagation only. However, the current DL MU-MIMO suffers from the high complexity and overhead, especially for long-range outdoor sensor networks. High station implementation complexity results from the following reasons: channel measurement requires complicated computation, immediate compressed beamforming report feedback requires fast computation, and receiver might need multiple receive antennas to cancel self-interference. Additionally, the sounding and feedback to obtain channel information for MIMO operation consumes medium time since the large amount of channel measurement feedback transmission is required. This channel measurement is only valid within coherent channel time. For low volume and infrequent sensor or meter types of data traffic, new channel measurement update and feedback at each active period would be required, results in high signaling overhead.
On the other hand, DL MU-MIMO transmission realized using multiple sectorized beams could lead to many simplifications with good spatial isolation. For one, infrequent channel update is sufficient. Another reason is that the feedback data can be significantly reduced (simplified). Currently, there is no solution to employ the signal format and protocol of DL MU-MIMO to enable the simultaneous transmission to multiple stations using sectorized beam operation. A solution of DL multi-sector transmission (MST) is sought.